This invention relates to circuitry for responding to electromagnetic signals and, more particularly, to probes for transmitting and/or receiving electromagnetic signals characteristic of signals emitted by nuclei in nuclear magnetic resonance analyses. Specifically, the invention is directed to a nuclear magnetic resonance probe having increased sensitivity, especially in circumstances where the sample or specimen under examination, such as biological tissue subjected to medical diagnosis, is a lossy dielectric material.
When nuclei possessing magnetic moments are placed in a strong magnetic field, they align and precess at their Larmor frequency, a characteristic frequency generally occurring in the radio frequency range. Particular nuclei can be excited by a burst of radio frequency (RF) electromagnetic energy at approximately their Larmor frequency. When the applied RF electromagnetic field is removed, the nuclei relax toward their equilibrium conditions, emitting RF signals which are characteristic of the molecular environments in which the nuclei reside. This phenomenon is known as nuclear magnetic resonance (NMR) which can be used for obtaining structural and dynamic information about the molecules of a sample.
In the past ten years, the application of NMR to biological examinations has proven to be enormously fruitful. In particular, the ability to study metabolism and produce images in vivo has assured increasing use of NMR in medicine. One of the major drawbacks of the NMR technique, however, has been its lack of sensitivity when compared with other spectroscopic techniques. It is widely accepted that much of the loss in sensitivity lies in the probe of the nuclear magnetic resonance analysis system and occurs in the tuned circuit which excites and detects the nuclei of interest. In accordance with the invention, an apparatus and method are provided which substantially increase the sensitivity of a nuclear magnetic resonance probe, especially when examinations are performed on biological tissue.
Exciting and detecting nuclear magnetic resonances generally requires a nuclear magnetic resonance probe having a tuned circuit which resonates at approximately the Larmor frequency of the nuclei. The circuit for known nuclear magnetic resonance probes resembles that shown in FIG. 1. See D. I. Hoult and R. E. Richards, J. Mag. Res. 24, 71-85 (1976), at page 81, FIG. 5. The capacitor C.sub.1 is used for tuning the probe circuit, while the capacitor C.sub.2 is used for impedance matching the circuit to external probe electronics. The losses in the probe circuit can originate in numerous places, the most elemental of them being the resistance of the sample coil L.sub.s itself. The resistance R.sub.p represents the parallel equivalent of the series resistance shown in FIG. 5 of Hoult and Richards and is proportional to the quality factor or Q of the circuit.
Other losses can be traced to losses in the sample. Usually, when a sample of biological tissue is introduced into the sample coil L.sub.s, rather large losses are incurred, and the circuit Q falls dramatically. Much of these losses can be attributed directly to the conductivity of the ions in tissue. The intracellular concentration of potassium ions within tissue is typically about 150 mM, while the extracellular fluid contains about 150 mM sodium chloride, and the conductivity of these samples causes radio frequency losses which can adversely affect the signal-to-noise ratio of the NMR spectra. Hoult and Lauterbur have identified these sample losses as either inductive or dielectric in nature. See D. I. Hoult and Paul C. Lauterbur, J. Mag. Res. 34, 425-433 (1979). Unfortunately, as Hoult and Lauterbur indicate, the sensitivity of the probe circuit is substantially degraded by these losses, varying roughly as Q.sup.1/2 at a fixed frequency.
Dielectric losses are indicated to be associated with the distributed capacitance of the sample coil L.sub.s and arise from lossy materials in the vicinity of the sample coil. Hoult and Lauterbur derive an expression which describes the losses of the probe circuit when the sample coil L.sub.s is completely immersed within a lossy dielectric. The effect upon the probe circuit is represented by the resistance R.sub.d and the capacitance C.sub.d shown in FIG. 2A connected in parallel with the sample coil L.sub.s.
D. G. Gadian and F. N. H. Robinson, J. Mag. Res. 34, 449-455 (1979), have modified the circuit shown in FIG. 2A to account for an arrangement often employed in the use of nuclear magnetic resonance probes, where the sample is contained in a glass tube which in turn is separated from the sample coil L.sub.s by an air gap and, therefore, insulated from the sample coil. In their circuit, shown in FIG. 2B, the capacitance C.sub.3 represents the distributed coil-sample capacitance of this air and glass insulation.
Inductive losses are also a consequence of the conductivity of the sample and originate from eddy currents induced in the sample by the alternating RF magnetic field. Little can be done to reduce such losses, except to make the sample less conductive. Cf. Donald W. Alderman and David M. Grant, J. Mag. Res. 36, 447-451 (1979).
The previous works indicate that the dielectric losses can be eliminated where the sample is contained in a glass tube by interposing a Faraday shield between the sample coil L.sub.s and the sample. The magnetic (or inductive) losses are intrinsic and, therefore, unavoidable. See P. Mansfield and P. G. Morris, Advances in Magnetic Resonance: NMR Imaging in Biomedicine, Supplement 2, Academic Press, New York (1982), at page 201.
Unfortunately, the previous works describing losses in various nuclear magnetic resonance probes have been found to inadequately describe the losses encountered in the analysis of lossy dielectric samples, such as biological tissue. In particular, the approach has been to treat the source of dielectric losses as occurring purely across the sample coil L.sub.s. What has gone unnoticed is that for purposes of shielding, whether or not the sample is in a glass tube, but primarily in the former case, the sample and probe circuit are generally contained in a metal enclosure to which the ground lead of the circuit is attached. The basis of the present invention is the discovery that additional dielectric losses associated with the distributed capacitances between the sample coil L.sub.s and the shield and other ground points of the probe circuit are present. Such dielectric losses are associated with the coil-to-ground impedance being decreased by the presence of the lossy dielectric sample within the sample coil L.sub.s, which provides an insidious current leakage path. In addition, in circumstances where the physical dimensions of the sample are comparable to a wavelength, radiative losses or transmission losses appear. The result is parasitic losses from the sample coil L.sub.s through the sample to ground, which increases the resistive loss of the probe circuit, thereby adversely affecting the sensitivity of the circuit. In accordance with the discovery of the invention, a tuning and impedance balancing apparatus and method are provided by which the influence of such parasitic losses is reduced, whereby the sensitivity of the probe circuit is increased.